1. Field of the Invention
This invention relates to scuba diving apparatus and particularly to a first stage regulator interposed between a tank of high pressure air and a second stage or demand regulator.
2. Prior Art
Demand breathing regulators are designed to operate efficiently when the supply pressure thereto is approximately 135 pounds per square inch. A supply tank easily accommodates breathable gas as to pressures of about 3,000 pounds per square inch. Accordingly, a first stage regulator is conventionally interposed between the tank and the demand regulator to reduce the tank pressure to the designed value. A particularly advantageous first stage regulator design is disclosed in U.S. Pat. No. 4,230,140 to Hart and assigned to the assignee of the present application. This first stage regulator provides a piston subjected on one side to ambient pressure and on the other to downstream air pressure. The piston is arranged to open when the ambient pressure overpowers the downstream pressure by a preset amount determined by a bias spring and to close when downstream pressure rises to the preset amount. Ideally, the pressure exerted by the spring over the effective area of the piston is equal to the intermediate pressure desired for the second stage regulator, such as 135 pounds per square inch. Accordingly, if the pressure in the stem drops below a value less than 125 pounds per square inch above ambient, a spring assisted by ambient pressure moves the piston away from a seated chamber until the ambient pressure and spring pressure are counter-balanced, whereupon the piston moves to a re-seated configuration. Unfortunately, the pressure in a conventional high pressure air tank typically varies from the beginning of the dive when it is maximum at about 3,000 pounds per square inch to the end of the dive when it is reduced to about 500 pounds per square inch or less. This variation of the pressure of the high pressure air tank during the dive, affects the intermediate pressure which varies in the same direction. In other words, when the pressure of the air tank is at maximum, the intermediate pressure is highest and as the air tank pressure is reduced during the dive, the intermediate pressure is also reduced. Thus for example, in the aforementioned prior art design, as the supply pressure varies from 3,000 PSIG to 500 PSIG, the intermediate pressure varies typically from 145 to 125 PSIG.
This reduction in intermediate pressure during the dive period creates a problem in that during the start of the dive, the intermediate pressure is the highest and accordingly, so is the flow rate of air to the second stage regulator. Some second stage regulators can become sensitive from the high intermediate pressure and either leak or freeflow. Both of these conditions can waste air and shorten the dive unnecessarily. Furthermore, toward the end of the dive period, the intermediate pressure is at the lowest, which can cause some second stage regulators to become harder to breath through because they require an increased cracking effort, due to reduced flow rates at the lower tank pressures. The problems associated with high pressure have been recently further exacerbated by improvements in tank material technology which permits tank pressures to be raised to as high as 5,000 PSIG. Accordingly, the need for overcoming the aforementioned disadvantages of the prior art have become even more urgent.